A key suspect in last month's Bangkok bombing has admitted he handed a backpack containing a bomb to a man in a yellow shirt who was later seen leaving a similar bag at a shrine just before the blast.

The account offered the first possible reconstruction of events leading to the August 17 attack at Erawan Shrine, which left 20 people dead and more than 120 injured.

Authorities say they still do not know the motive but are also reluctant to call the blast an act of terrorism, which would harm Thailand's image as a tourist destination.

Police have arrested two foreigners, raided two apartments on the outskirts of Bangkok where they confiscated bomb-making materials, and say they are looking for 10 other suspects.

Several Turkish and Chinese links to the blast have boosted a theory that the bombing was to avenge Thailand's forced repatriation of more than 100 ethnic Uighurs to China in July. Uighurs are related to Turks, and Turkey is home to a large Uighur community.

The Erawan Shrine is especially popular with Chinese tourists, feeding the idea that it could be a target for people who believe the Uighurs are oppressed by China's government.

A man wearing a yellow T-shirt is seen near the Erawan Shrine before the explosion in Bangkok (Royal Thai Police via AP)Police investigators work at the Erawan Shrine the morning after the explosion in central Bangkok (AP)Police, soldiers and other officials at the Erawan Shrine the day after an explosion in Bangkok (AP)

One of the men in custody, identified as Yusufu Mierili, was arrested on September 1 near the Thai-Cambodia border carrying a Chinese passport and has confessed to playing a key role in the attack.

The passport indicated he was from China's western region of Xinjiang, home to the Turkic-speaking Uighurs. His DNA and fingerprints were found in the two apartments police raided, including on a container of gunpowder in one of the apartments.

On Tuesday, police took Mierili back to the apartments and also to a chemical supply shop where he said he bought "four substances" used to make the bomb, national police spokesman Prawut Thavornsiri said. He declined to say what the materials were or if Mierili is believed to have made the bomb himself. He said the shop owner remembered Mierili as a customer.

A second day of re-enactments continued, when police and armed commandos led Mierili in handcuffs and body armour to Bangkok's main train station and then to the site of the blast at Erawan Shrine, about 3 miles away.

Public re-enactments in front of the media are a common part of Thai criminal investigations, although they have been criticised for implying a suspect's guilt before a trial.

Mr Prawut described Mierili as "cooperative", and during the re-enactment he could be seen pointing things out to police and at times laughing with them.

This image released by the Royal Thai Police shows a detailed sketch of the main suspect in a bombing that killed 20 people at the Erawan shrine in downtown Bangkok, on Monday(Royal Thai Police via AP)

Mierili told police that on the day of the attack, he picked up the backpack from one of the apartments in the Bangkok district of Nong Chok and then took a three-wheeled tuk-tuk taxi to Hua Lamphong train station, Mr Prawut said.

"Yusufu said the backpack that he carried was heavy and it was a bomb," he added.

The suspect said he received instructions from "someone" on WhatsApp, a mobile messaging app, telling him to deposit the backpack at a bench outside the station and wait for another person who would pick it up and leave him another backpack in return, Mr Prawut said.

"This place is where he met with the yellow-shirted man to exchange backpacks," Mr Prawut told reporters outside the station. He said Mierili told police he "picked up the backpack that the yellow-shirted man left for him, and walked away".

The yellow-shirted man is believed to be the bomber, and police say they think he fled the country. He was seen in surveillance camera video placing a large, black backpack at the shrine and leaving minutes before the blast occurred.

After the train station hand-off the two men went their separate ways to Erawan Shrine, where Mierili said he was told to take pictures during the blast, Mr Prawut said.

He said Mierili showed police where he stood to watch the explosion - in a plaza in front of Central World, a popular shopping centre.

Mierili, who police have also identified as Yusufu Mieraili, faces charges of conspiracy to possess unauthorised explosives.

Another suspect, who was arrested on August 29 at one of the two apartments, possessed a fake Turkish passport when he was detained. They have identified him as Adem Karadak.